This invention relates to a process for treating surfaces of metal such as copper, said metal being subjected to a pre-treatment of copper conductor of wiring boards for inner layers which are bonded via prepregs in the production of multilayer printed wiring boards.
Multilayer printed wiring boards are usually produced by the following processes:
(1) One process comprises forming printed copper conductors by etching of copper foils of copper clad laminates to give printed wiring boards for inner layers, treating surfaces of copper conductors with a chemical solution of ammonium persulfate, cupric chloride, copper acetate, or the like for roughening, laminating and adhering copper clad laminates for outer layers via prepregs (products of impregnating base materials such as paper and glass fabrics with thermosetting resins, followed by drying them) on one or more thus treated inner layer printed wiring boards, drilling or punching through holes at predetermined portions including the copper conductors of inner layers, immersing in a sensitizing solution, forming electroless copper plated layers on walls of the through holes, and etching the copper foils for outer layers in accordance with the inner copper conductors pattern to form outer layer copper conductors.
(2) Another process comprises subjecting one or more copper clad laminates having both-side roughened copper foils for improving adhesiveness and the like (both-side roughened copper foils) to the same treatment as in the process (1) to give printed wiring boards for inner layers, omitting the surface roughening step, laminating and adhering copper clad laminates for outer layers via prepregs on one or more thus treated inner layer printed wiring boards, followed by the same procedures as described in the process (1) to give a multilayer printed wiring board.
(3) A further process comprises forming printed wiring boards for inner layers in the same manner as described in the process (1), roughening copper conductor surfaces in the same manner as described in the process (1), treating with a sodium chlorite-sodium bydroxide-trisodium phosphate treating solution, forming copper oxide films (1-3 .mu.m) on the roughened copper conductor surfaces, laminating and adhering copper clad laminates for outer layers via prepregs on one or more thus treated inner layer printed circuit boards, followed by the same procedures as described in the process (1) to give a multilayer printed wiring board.
But these processes have following disadvantages.
The multilayer printed wiring board produced by the process (1) is insufficient in adhesive strength between an inner layer copper conductor and a resin layer obtained by curing the prepseg. Further in the step of mounting various electronic devices on the produced multilayer printed wiring board with solder, there take place peeling between an inner layer copper conductors and a resin layer obtained by curing the prepreg, and sometimes blisters derived from the peeling which can be identified by the naked eye from outside.
In the process (2), since the both-side roughened copper foil is used, a roughened surface is sometimes scratched in the steps until the formation of printed wirings on the copper clad laminates. In such a case, the scratched portion is lowered in adhesive strength. Further, since the roughened surface of copper foil is used, pattern definition of printing etching resist or pattern definition of etching resist in a UV photographic printing method used for forming printed wiring patterns is poor.
In the process (3), the copper oxide film is dissolved by the sensitizing solution used in the step of providing a plating catalyst on through hole surfaces for forming electroless copper plated films thereon (the sensitizing solution being an acidic solution of pH 4 or less) or by an acidic solution used in the step for removing a smear adhered to a cut surface of inner layer copper conductors after drilling through holes and the acidic solution permeates between inner layer copper conductor surfaces and resin layers to bring about a pink-ring phenomenon and to lower the adhesiveness between an inner layer copper conductor surface and a resin layer obtained by curing the prepreg and insulation resistance between conductors.